Also, if you're the one starting the band (ie you're assuming a leadership position) it helps to have material somewhat worked out before you start looking. If you're doing covers, pick a few out and put them as examples on your ad, AND be prepared to play them (and provide parts or recordings for potential members). Same deal with originals, have at least a few tunes worked out and "ready". If you want to do originals but you don't have anything written yourself, probably it's a better idea to either join a group, or wait a bit and work some songs out yourself before you start advertising.
If I'm looking for a band, I want to be able to jump in and get to work. If things aren't somewhat organized, and there's no clear direction, or every practice starts out as "So what are we working on? I dunno!" that's a problem and I won't be there long.
This is probably more than most high schoolers are looking for when they get involved in a band, but the more prepared you are to run the band, the better off you'll be.
Annnnd, use a template or something if you aren't graphically inclined! A professional looking poster goes a long way, if I see a handwritten xerox copied flyer I run away
